In the electronics industry it is often desirable to cover or coat an existing refractory metal surface with a brazable or solderable surface. Applications for such a procedure, include but are not limited to, I/O pads, wire bond pads, C4's (Controlled Collapse Chip Connection), seal bands, to name a few.
Many methods are available and practiced in the industry to cover or coat an existing refractory metal surface with a brazable or solderable surface. The most commonly used approach for treating refractory metal surfaces in the microelectronic packaging business is to employ electroplating or electroless-plating of pure or substantially pure Ni (nickel) film from an aqueous bath which is at or near room temperature.
Nickel is generally the metal of choice for plating refractory metals because it can be made to bond well with any of the refractory metals. In addition, Ni possesses good wetting characteristics for subsequent bonding processes, such as brazing or soldering, and it has excellent corrosion characteristics.
Recently, a few high temperature, dry, halide transfer processes have been disclosed and subsequently used by the industry for the purpose of plating nickel on molybdenum or tungsten.
One method disclosed in U.S. Pat. No. 4,590,095 (Park) uses a pack cementation approach. The essential elements for pack cementation are a powder metal source, an activator, and an object to be plated. Basically, the elements are placed in a chamber and the object is buried in a mixture of the powder metal source, activator, and usually an inert ceramic powder, such as, alumina, and then heated to a high temperature to establish vapor transport. The process allows for mass transfer of the gas species. For the Park process pure nickel powder was used as the metal source and the activator used was ammonium iodide.
A departure from this pack cementation approach for a halide transfer process was disclosed in U.S. Pat. No. 4,664,942 (Park). In this case ammonium iodide and pure nickel were still the essential elements for the halide transfer process. However, in this case nickel screens were used as the metal source rather than the nickel powder. And, the objects to be plated, containing exposed surfaces of refractory metal, were placed in stacks with the nickel screens acting as separators in the reaction vessel or work boat. The ammonium iodide activator for the process was held in a separate crucible within the work boat. The elements were again heated to a high temperature to establish vapor transport. The open nickel screen allowed for mass transfer of the gas species and also served as the nickel source.
Most recently, another improvement was put forward in U.S. patent application Ser. No. 08/668,295 (Reddy et al.), filed on Jun. 21, 1996, entitled "CVD OF METALS CAPABLE OF RECEIVING NICKEL OR ALLOYS THEREOF USING IODIDE", presently assigned to the assignee of the instant Patent Application, and the disclosure of which is incorporated herein by reference, where CuI was disclosed as a preferred iodide activator providing various advantages.
In general, for halide transfer metal deposition processes, it is very desirable that the solid metal source material and the metal area to be plated are kept in close physical proximity to each other. This condition is necessary in order to maintain a reasonable rate of metal deposition during the process.
In each of the known halide transfer processes, the metal source material, powder or screens, were kept in close physical proximity to the refractory metal surface to be plated, however, in addition, due to the specific geometrical configuration of each assembly, the metal source material can at least at some point, also come into direct physical contact with the metal surface to be plated. It has been discovered that when the source metal and the target areas do touch each other, during the deposition process, while using any of the known processes, they, the source and the sink, can weld together and form a bond. When the nickel plated part and the other assembly materials are subsequently separated, after the deposition process has been completed, a defect in the deposited nickel film can be readily observed. This defect can take the form of a taffy pull of metal or a piece of metal debris or a missing section of the deposited nickel film, etc. This condition will normally, result in the disqualification of the part or work piece.
The present invention, however, teaches a new method for configuring a work boat in a halide transfer process where nickel metal is electrolessly deposited onto a refractory metal surface. With the method of this invention the metal source and the refractory metal surface to be plated, are kept in close physical proximity, as required, to effect rapid deposition rate, but where at least one inert stand-off is also in contact with the receiving metal surface such that the source metal and the surface being plated are maintained in complete physical isolation from each other. With this invention there does not exist any opportunity for the source and the sink to touch and weld and form a bond. Consequently, the disqualifying defect, that appears when the metal source and the refractory metal are in contact, which happens in the prior art processes is totally avoided.